Northern Line to stay closed

Key sections of the Tube's Northern line will be closed until next week "at the earliest", it emerged today.

Sunday's derailment at Camden Town, in which seven passengers were injured, has caused far more damage than first thought. It means continuing travel chaos for the Northern line's near-700,000-a-day users.

A London Underground spokesman said today: "We do not expect to restore full services on the Northern line before next week at the earliest."

The sixth carriage of the train which came off the track was removed last night. Only today have engineers been able to assess the full amount of repair work which needs to be done.

The spokesman added: "Signals, cables and other equipment were badly damaged."

It is understood that 50 signal cables were ripped from their mountings. Restoring and testing the signals is the principal reason why services cannot be restored before Monday.

Tim O'Toole, LU managing director, said: "I am very sorry for the disruption that the current service suspension is causing to our customers. But we must be assured of the safety of this section of the line before we resume running trains through it.

"Until I have that assurance from Tube Lines [the private sector consortium responsible for maintenance at Camden Town], and from my own engineers, we cannot restore the service."

The Northern line remains closed through central London. LU has laid on 150 buses which will call at all stations still closed.

Transport Secretary Alistair Darling said today the derailments at Camden Town and, last Friday, at Hammersmith, were of "deep concern" - but he denied the Government's drive for partprivatisation was responsible.

Maintenance schedules had not changed since private companies took over routine repairs, he insisted. Tube safety was now the responsibility of London Mayor Ken Livingstone.

However, anxiety over safety standards was fuelled today as it emerged that the cracked rail which led to the Hammersmith derailment, on the Piccadilly line, may have been broken for five months.

Maintenance workers from the private consortium Metronet failed to spot the break during a visual inspection the previous evening. But they were cleared today of having failed to do the job properly. Tim O'Toole, LU's managing director, said: "The [track inspection] team was vigilant-They reported other faults that night."

He said the crack was "almost microscopic", had "migrated up from the bottom of the rail" and "would not have been visible to the naked eye".

At present track inspections are carried out between every 24-72 hours. Ultrasonic testing, which uses a radio beam to seek out faults with the rail, is carried out only every six months.

LU is planning to increase the use of track inspections using highly sensitive ultrasound equipment to prevent similar incidents.

The Hammersmith track was last inspected using ultrasound equipment in May. Another inspection was due next month.

Mr O'Toole said: "We will now have to look at increasing the frequency of ultrasonic testing."

The RMT, largest of the Tube unions, has warned it will ballot for industrial action unless LU agrees by Friday to instigate 24-hour inspections of the track across the network.